1. Field of the Invention
The invention relates to micro-display engines, particularly for portable displays, such as head-mounted viewers. Typically, the micro-display engines feature compact image generators that output real images and magnifying optics that produce enlarged virtual images.
2. Description of Related Art
Micro-display engines can be incorporated into a variety of devices and are particularly suitable for portable devices including hand-held, wrist-worn, or head-mounted devices. For example, the micro-display engines can be used in pagers, wireless mobile telephones, or wrist-worn computers or can be mounted in various types of viewers for displaying data, graphics, or video.
Among the known devices, single micro-display engines are used in monocular viewers or biocular viewers, which provide dual images of the same image sources, and pairs of micro-display engines are used in binocular viewers. All three types of viewers can be head-mounted (i.e., mounted on an observer's head), which are intended for viewing only by the wearers of the displays.
The micro-display engines include compact image sources and optical focusing systems for magnifying the images produced by the sources. The usual outputs of the image sources are small but high-resolution real images. The focusing systems magnify the high-resolution images as virtual images within a desired field of view. Both the image sources and the focusing systems are made as small and lightweight as possible to minimize the effect of their presence on the user.
Reflective focusing optics are preferred over refractive lens systems to avoid chromatic aberrations. The apparent field of view tends to increase as the focal length decreases. Magnification of a virtual image also tends to increase as the object distance approaches the focal length. Both requirements can be met with a single reflective focusing optic (e.g., a concave mirror), that can be positioned close to both the observer's eyes and the image source. Typically, the reflective focusing optic is aligned with the observer's eyes and is optically coupled to the image source through a beamsplitter. The beamsplitter reflects image light from the image source to the reflective focusing optic and transmits the image light from the reflective focusing optic to the observer's eyes.
The micro-display engines can be of the immersive or non-immersive type. Immersive displays block the transmission of ambient light in alignment with the view of the display. Non-immersive displays allow ambient light to transmit through the displays in alignment with the view of the display. For example, objects in the observer's environment can be seen through some non-immersive virtual displays, including such displays that present information about the observer's environment. Other non-immersive displays admit light so the observer can remain oriented to the environment that would otherwise be in view or for purposes of background lighting. Either type of display, immersive or non-immersive, can be enclosed in a full or partial housing (also referred to as a shroud) to block the admission of unwanted ambient light within the field of view.
Ambient light in the viewing direction is typically admitted into the imaging systems of non-immersive displays employing reflective focusing optics through the reflective focusing optics themselves. This requires the reflective focusing optics to be at least partially transmissive to admit ambient light and at least partially reflective to magnify the image source. The partial reflectivity results in the loss of some light from the image source. The partial transmissivity results in the loss of some on-axis ambient light. The loss of image light is inefficient, requiring a higher output image source. The loss of ambient light may also be of concern. Views transmitted by ambient light through the partially reflective focusing optics can also be undesirably distorted by passage through the partially reflective focusing optics.
Transmissive liquid crystal displays (LCDs) are among the preferred image sources. Transmissive LCD's typically interleave a polarization varying layer containing liquid crystals between two polarizer layers. Light is admitted through the first polarizer layer and images produced by the LCDs appear on the second polarizer layer. It is the real image appearing on the second polarizer layer that is magnified, generally as a virtual image, by the focusing optics. As such, the imaged surface of the second polarizer layer must be made optically correct to avoid the imaging of artifacts on the second polarizer layer or the introduction of distortions, aberrations, or other disturbances into the image, which adds considerably to the optical requirements and cost of the second polarizer layer.